Aspartate Carbamoyltransferase - Allosteric Site

Allosteric Site

The allosteric site in the allosteric domain of the R chains of the ATCase complex binds to the nucleotides ATP, CTP and/or UTP. There is one site with high affinity for ATP and CTP and one with 10- to 20-fold lower affinity for these nucleotides in each regulatory dimer. ATP binds predominantly to the high-affinity sites and subsequently activates the enzyme, while UTP and CTP binding leads to inhibition of activity. UTP can bind to the allosteric site, but inhibition of ATCase by UTP is possible only in combination with CTP. With CTP present, UTP binding is enhanced and preferentially directed to the low-affinity sites. On the converse, UTP binding leads to enhanced affinity for CTP at the high-affinity sites and inhibits enzyme activity by up to 95%, while CTP binding alone inhibits activity to 50% to 70%. Comparison of the crystal structures of the T and R forms of ATCase show that it swells in size during the allosteric transition, and that the catalytic subunits condense during this process. The two catalytic trimers move apart along the threefold axis by 12 Å, and they rotate about this axis by 5° each, ultimately leading to a reorientation of the regulatory subunits around their twofold axis by 15°. This quaternary structure change is associated with alterations in inter-subunit and inter-domain interactions. The interaction between subunits C1-C4 and R1 is extensively modified during this conversion. In particular, there is large movement of amino acid residues 230-254, known collectively as the 240s loop. These residues are located at the cleft between the carbamoyl phosphate and aspartate domains at the C1-C4 interface. The overall outcome of these structural changes is that the two domains of each catalytic chain come closer together, ensuring a better contact with the substrates or their analogues.

During this structural transition, some interactions between side-chains are lost and some others are established. Studies have confirmed that the position of the 240s loop directly affects substrate binding in the corresponding active site. Earlier studies using site-directed mutagenesis of the 240s loop showed that interactions between Asp271 and Tyr240, and between Glu239 of C1 and Tyr165 of C4 would stabilize the T-state, while interactions between Glu239 of C1 and both Lys164 and Tyr165 of C4 would stabilize the R-state.

Located close to the 240s loop and the active site, the loop region encompassing residues 160-166 plays a role in both the internal architecture of the enzyme and its regulatory properties. In particular, the residue Asp162 interacts with Gln231 (known to be involved in aspartate binding), and binds the same residues in both the T and R states. A mutant that had this residue mutated to alanine showed a huge reduction in specific activity, a two-fold decrease in the affinity for aspartate, a loss of homotropic cooperativity, and decreased activation by ATP. It was suggested that the change in the overall structure caused by the introduction of this residue affects other residues in the R1-C1, R1-C4 and C1-C4 interfaces, which are involved in the quaternary structure transition.